<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #box{
            position: relative;

        }
        img {
            width: 200px;
            height: 200px;
        }
        ul{
            width: 100px;
            position: absolute;
            top: 150px;
            left:20px;
        }
        li{
            list-style: none;
            width: 5px;
            height: 5px;
            background-color: red;
            float: left;
            margin: 0 10px 0 10px;
            border-radius: 50%;
            cursor: pointer;
        }
    </style>
    <script>
        window.onload =function(){
            var pic1 = document.getElementById("pic1");
            var pic2 = document.getElementById("pic2");
            var pic3 = document.getElementById("pic3");
            var im = document.getElementById("im");
            pic3.onmousemove = function(){
                im.src = "images/3.jpg";
            }
            pic2.onmousemove = function(){
                im.src = "images/2.jpg";
            }
            pic1.onmousemove = function(){
                im.src = "images/1.jpg";
            }
        }
    </script>
</head>
<body>
    <div id="box">
        <a href="javascript:;"><img src="images/1.jpg" alt="" id="im"></a>
        <ul>
            <li id="pic1"></li>
            <li id="pic2"></li>
            <li id="pic3"></li>
        </ul>
    </div>
</body>
</html>